This invention generally relates to a ductile cast iron or spherulitic graphite cast iron and, more particularly, to the ductile cast iron having an improved resistance to oxidation at high temperatures and an improved resistance to thermal fatigue. The ductile cast iron, which is also referred to as spherulitic graphite cast iron, according to this invention exhibits a high resistance to both oxidation at high temperatures and thermal fatigue when used as a material for an automobile exhaust manifold.
As is well known, an automobile exhaust manifold, i.e., the piping through which high temperature exhaust gases discharged from an automobile combustion engine flow, tends to be alternately heated and cooled, receiving a frequent thermal shock. Therefore, the automobile exhaust manifold is generally required to have a relatively high resistance to oxidation at high temperatures and also a relatively high resistance to thermal fatigue. In particular, the resistance to oxidation is an important property for the suppression of the growth of an oxide layer and the improvement on the peel resistance of the oxide layer. The failure to have a capability of suppressing the oxide layer and a high peel resistance tends to result in separation of oxide scales which would, when the exhaust system of the automobile engine is held under negative pressure such as occurring during the transit period in which the fuel intake and exhaust valves are simultaneously opened, be sucked towards the engine cylinder. Once this happens, the oxide scales so sucked will constitute a cause of accelerated wear of the valve member, the valve seat and the internal surface of the engine cylinder.
In view of the above, a high resistance to oxidation at high temperatures is an essential property which a material for the exhaust manifold must have.
Hitherto, as a metallic material excellent in resistance to oxidation at high temperatures, there has been well known a ductile cast iron which exhibits a ferrite structure as cast and contains carbon in an amount of 3.3 to 4.0 wt%, silicon in an amount of 3.5 to 4.5 wt%, phosphorous in an amount of 0.04 wt% or less, manganese in an amount of 0.3 wt% or less, sulfur in an amount of 0.01 wt% or less, and magnesium in an amount of 0.02 to 0.04 wt%. The ductile cast of the above described composition is disclosed in, for example, the Japanese Patent Publication No. 54-38968 published Nov. 24, 1979, and is described as suitable for the production of automobile exhaust manifolds.
In this prior art ductile cast iron, since silicon is contained in an amount within the range of 3.5 to 4.5 wt%, which silicon forms a protective layer of SiO.sub.2, the amount of oxide scales formed is minimized, and since the content of any one of phosphorous, manganese and sulfur is relatively small, the cracking would not tend to occur readily although it can not be avoided to such an extent as to make the cast iron utilizeable in practical production.
Despite the advantage in that, since the content of silicon is relatively great, the amount of oxide scales formed can be minimized, the employment of a relatively great amount of silicon such as within the range of 3.5 to 4.5 wt% renders the matrix so fragile that the thermal fatigue characteristic thereof is considerably lowered.